I. Field of the Invention
This invention relates to an improved copolymer latex binder and a process for the preparation thereof. This invention is also directed to a process for preparing chemically bonded, nonwoven fiber material using the copolymer latex binder herein.
II. Description of the Prior Art
Nonwoven fiber materials, generally prepared by randomly depositing individual fibers to form a web, impregnating the web with a binder to hold the individual fibers together, and then heating the web at a set temperature, are known to possess distinct advantages over conventional woven materials. Among the advantageous properties characterizing the nonwoven fiber materials are absence of ravelling, smoother surfaces, increased softness, improved hand, greater absorbency, higher loft, and the like.
The binders used to impregnate nonwoven webs are typically aqueous dispersions of copolymer latices which form rubbery or elastic films upon drying. A wide variety of copolymers are known in the art to be used in latex binders. Typical of these copolymers are butadiene copolymers such as butadiene-styrene-acrylonitrile, alkyl acrylate copolymers, rubbery chloroprene copolymers, vinyl and vinylidene ester copolymers, and the like. These copolymers may contain (i.e., be prepared with) any of a wide variety of polymerizable and/or crosslinkable comonomers such as, for example, substituted alkyl acrylate esters such as 2-hydroxyethyl acrylate; vinyl halides; vinyl esters; ethylenically unsaturated amides such as acrylamide and N-methylolacrylamide; and the like. Typical copolymers of this type employed in latex binders for nonwoven fiber material are described in, for example, U.S. Pat. Nos. 3,985,929; 3,231,533; and 2,984,588.
When a nonwoven web impregnated with a latex binder is dried, the latex particles that were evenly distributed throughout the web on wet application may move to the surface of the web, resulting in nonuniform distribution of the latex throughout the web and lowered physical properties, manifested generally in an increase in the fabric hand. In extreme cases, such binder migration can cause delamination of the treated web into two separate pieces.
Binder migration is favored by thick webs, low binder-to-water ratios, sudden rapid heat increase, and/or very stable latex binders. Efforts to control migration by inhibiting binder movement include adding thickeners or heat-sensitizing agents to the binder either before or after application of the binder to the web. Heat-sensitizing agents inhibit binder migration by lowering the temperature at which coagulation, or phase inversion, occurs in the binder.
Early examples of heat-sensitizing agents are emulsified salts that dissolve when the temperature is raised electrolytic salts such as zinc salts and ammonium and multivalent salts (e.g., aluminum sulfate) for anionic latex binders; cationics such as quaternary amines; and surfactants or protective colloids having inverse solubility, e.g., organic polysiloxanes, polyvinyl methyl ethers, etc. This latter group of compounds functions by precipitating out of the latex as the temperature is raised (see J. R. Holker, "Formed Fabrics Industry", June, 1976, pp. 26-30).
Many of the heat-sensitizing agents of the prior art, however, prove to be unsatisfactory in one or more respects. Thus, a nonwoven web impregnated with a latex binder containing certain of the early sensitizing agents is not sufficiently resistant to solvent, may easily swell in plasticizers, or may have nonuniform bonding of the latex. Furthermore, when an electrolytic salt is used as heat-sensitizing agent, depending on the particular copolymer and the amount of salt added, the binder itself may become destabilized upon addition of salt and coagulate at room temperature. For example, it is well known that copolymer latex systems based on butadiene-styrene-acrylonitrile copolymers are rendered mechanically unstable and coagulate at room temperature upon addition of a salt thereto. In contrast, there are latex binders for nonwoven materials which are stable upon addition of up to large amounts of a salt thereto, i.e., they will not coagulate at any temperature (see U.S. Pat. No. 3,925,288). The salt-containing latex binders which coagulate with heat, however, do not have a definite coagulation temperature and undesirably tend to coagulate over a wide range of temperatures.
An acrylate-based polymer latex containing anionic surfactant wherein multivalent salts or cationics such as quaternary amines are used as heat-sensitizing agents is taught by D. Strong and J. Brodnyan in TAPPI, Vol. 58, No. 11, November, 1975, pp. 133-135. In this process the salt must be added to the web after the web has been impregnated with the latex binder. If the salt is added to the latex binder prior to impregnation of the web, the latex becomes mechanically destabilized and thus useless for impregnating purposes. In addition, if the multivalent salt or amine is used in amounts greater than about 5%, the latex will coagulate at room temperature.
New heat-sensitizing agents and improvements in prior art sensitizing agents have been discovered. For example, addition of certain organic acids to latex binders containing silicone polyether allows for the use of lesser amounts of latex binder (see U.S. Pat. No. 3,714,101). Relatively new heat-sensitizing agents for latex binders include functional propylene oxide-ethylene oxide block copolymers and propylene oxide adducts of triethanolamine or ethylenediamine (see German Pat. No. 2,263,921). However, there is a need in the art for a simple, relatively inexpensive heat-sensitizing agent which stabilizes a latex binder at room temperature but causes coagulation of the binder upon heating to a well-defined temperature. The limitation on the use of simple electrolytic salts as heat-sensitizing agents for latex binders, due to their destabilizing or overstabilizing effects mentioned above, has not been overcome in the prior art.
Accordingly, it is an object of the present invention to provide an improved copolymer latex binder containing a heat-sensitizing agent which binder is mechanically stable at room temperature.
It is another object to provide an aqueous copolymer latex binder for nonwoven webs which coagulates upon heating to a definite temperature below the drying temperature of the web.
It is further object to provide a heat-coagulate copolymer latex binder which, when used to impregnate nonwoven webs, does not migrate to the surface of the web during the drying operation.
It is a still further object to provide a process for preparing a heat-coagulable copolymer latex binder which can be used to prepare chemically bonded, nonwoven fiber material.